Space-time heterogeneities of the zooplankton distribution in La Concepción reservoir (Istán, Málaga; Spain)

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1 Hydrobiologia 455: , Kluwer Academic Publishers. Printed in the Netherlands. 157 Space-time heterogeneities of the zooplankton distribution in La Concepción reservoir (Istán, Málaga; Spain) M. J. Fernández-Rosado & J. Lucena Departamento de Ecología de la Facultad de Ciencias, Universidad de Málaga, Campus de Teatinos, Málaga, Spain Received 26 March 1998; in revised form 4 April 2001; accepted 20 April 2001 Key words: zooplankton, distribution, cycle, heterogeneity, density Abstract Space-time distribution of the zooplankton (Rotifera, Cladocera and Copepoda) density along the central axis (horizontally and vertically) and the shores of La Concepción reservoir have been studied for one year. Results show that the distribution along the central axis and the shores follows similar patterns both in space and time. Rotifera and Copepoda mainly appear during the first part of the mixing period and Cladocera during stratification. Horizontal distribution is characterized by the occurrence of density gradients from the intake to the dam. Vertical distribution present density maximum at the Secchi depth during mixing and at two times Secchi depth, during stratification. Zooplankton distribution is related to physical-chemical and biotic factors. Introduction Most studies on zooplankton distribution analyze temporal distribution (Wölfl, 1991; Arcifa et al., 1992; López-Adrian & Herrera-Silveira, 1994; Vasconcelos, 1994; Mayer et al., 1997), horizontal distribution (Armengol, 1984; Mavuti & Litterick, 1991; Viljanen & Karjalainen, 1993; Thys et al., 1998; Zsuga, 1998), horizontal and vertical distribution (Miracle, 1976; Soto et al., 1984; Pinel-Alloul, 1995) or littoral distribution (Sharma & Pant, 1984; Kairesalo & Penttilä, 1990; Lair et al., 1993; Zingel & Ott, 2000). Temporal variations in zooplankton distribution have been studied for a long time, but research on spatial patchiness has received less attention (Tonolli, 1949 a; Dumont, 1967). Currently, it is known that zooplankton spatial distribution are as heterogeneous as those of terrestrial and aquatic plants and animals (George, 1974; Malone & McQueen, 1983; Pinel-Alloul et al., 1988). Limnologists have related zooplankton distributions patterns to temperature, dissolved oxygen, ph, transparency, wind, social aggregates, water turbulence, trophic gradient and phytoplankton (Dumont et al., 1973; Dagg, 1977; Berzins & Pejler, 1989; Hanazato, 1991; Geller et al., 1992; Hanazato, 1992; Hart, 1994; Tallberg et al., 1999; Locke & Sprules, 2000). In reservoirs, the water turbulence degree that determines the existence of mixing and stratification periods and the water renewal that dilutes and horizontally transports the zooplankters is especially important (Zingel & Ott, op. cit.; Kvam. & Kleiven, 1995; Seda & Machacek, 1998). Water movement influence is more marked in organisms with small body-size, low reproductive rate and low mobility (Threlkeld, 1982; Armengol, 1984; Dirnberger & Threlkeld, 1986; Armengol et al., 1988). Finally, studies have demonstrated that zooplankton communities are also regulated by interspecific competition and predation (Brooks & Dodson; 1965; Dumont, 1972; Dodson, 1974; Gilbert, 1988; DeMott, 1989; Urabe, 1990; TilZer, 2000). Here, we simultaneously consider distribution in time and space (horizontal vertical) along the central axis and the shores, to construct a dynamic picture of zooplankton distribution. Along the annual cycle, we differentiated the mixing distribution and the stratification distribution, due to the different limnological characteristics of each period. We have also take into consideration the influence of abiotic and biotic factors.

2 158 Figure 1. Location of La Concepción reservoir and sampling stations. Materials and methods Study area La Concepción (Fig. 1) is an eutrophic reservoir located on Verde river (Istán, Málaga, Spain: N, 4 56 W). It has an area of 2.92 km 2, a capacity of 59 hm 3, a maximum depth of 68 m, a length from river intake to dam of 4.6 km and a maximum width of 1 km. Except for the intake, which is mixed all the year, the reservoir is monomictic (Fig. 2.A), with mixing from November to March and stratification from April to October. Dissolved oxygen and transparency also vary from the intake to the dam (Fig. 2). Chlorophylla distribution is similar in the central axis and along the shores, reaching highest concentrations in November and April, at the intake and at two times Secchi dept (Fig. 3). Phytoplankton is constituted basically by diatoms during mixing and by dinoflagellates, during stratification. (Lucena & Rodríguez, 1984; Fernández- Rosado et al., 1994). The shores of the reservoir have no plants. Sampling description Sampling was carried out between and hours G.M.T., from November 1987 to October 1988, every fortnight, on a transect of six stations along the central axis and near each shore, E1 being the intake station and E6 the dam station (Fig. 1). Samples in the central water column were taken at the surface (0 m), Secchi disk depth (S), at two times Secchi dept (2S) and bottom (B), because these points were considered as singular in the phytoplankton distribution in this reservoir (Algarra, 1982; Gálvez, 1986). This strategy assumes 10% and 1% of surface irradiance at S and 2S depths, respectively (Margalef et al., 1976; Algarra, 1982; Gálvez; 1986), and was verified in the field by means of a LICOR spectroradiometer model Li Shore samples were taken at the surface, approximately at 1.50 m from land. Water was collected using two 6-l Van Dorn bottles; next, filtered through a 45 µm NYTEX plankton net and fixed with 5% formol. The species identification and sample counts, as number of individuals per litre, were made using a NIKON microscope at ; for each sample, the number of 1-ml subsamples analyzed was determined by a variation of less than 10%. Nauplii of copepods were considered to constitute one group. Data processing In order to show spatial and temporal heterogeneities, zooplankton density data were logarithmically transformed and examined using ANOVA. First, annual cycle data in the central axis were processed by a time-transect-depth ANOVA. Afterwards, intake station (E1) data were analyzed by a time-depth ANOVA and data of the stations with mixing-stratification cycle (E2 E6), by a time-transect-depth ANOVA for each period. Finally, shores and central surface data were analyzed altogether in another ANOVA. In all these analyses, distributions with F s significant for p<0.05 were considered as heterogeneous; in such a case, the T-method (Tukey, 1951) was applied to compare two mean densities adjacent in time, transect or depth to detect the segregation between them. Species densities were correlated with environmental factors (temperature, oxygen and chlorophyll). In spatial distributions, heterogeneity degree, expressed as ANOVA mean squares, was correlated with population density. Finally, a principal component analysis was applied to logarithmically transformed densities of the whole reservoir and during the whole year, to differentiate interspecific associations in the zooplankton community. Components obtained were correlated with environmental factors (temperature, oxygen and chlorophyll) and with space-time localization (time, transect and depth). Variables that express space-time localization were codified assigning increasing values from the first date to the last one, from the intake station to the dam station and from the surface to the bottom. In this way, positive correlations with localization in time, transect or depth mean a preferential localization at the end of the cycle, at the dam or at the bottom; whereas negative correlations mean a prefer-

3 Figure 2. (A) Space-time distribution of temperature ( C) in each station. -o- =Secchi disk depth. (B) Space-time distribution of dissolved oxygen (mg l 1 ) in each station. 159

4 160 Table 1. F s values resulting from ANOVA application to zooplankton densities of shores and central surface. They are non significant for p<0.05 ROT: Polyarthra gr. vulgaris-dolichoptera 0.36 P. euryptera 0.70 Synchaeta gr. tremula-oblonga 0.22 S. pectinata 0.26 Anuraeopsis fissa 0.52 Hexarthra mira 0.20 Ascomorpha saltans 0.25 Asplanchna gr. girodi-brightwelli 0.13 Trichocerca similis 0.92 Collotheca gr. pelagica 0.50 CLA: Bosmina longirostris 1.61 Diaphanosoma mongolianum 2.16 Daphnia longispina 0.69 Ceriodaphnia pulchella 1.70 C. reticulata 2.00 COP: Acanthocyclops robustus 2.10 Tropocyclops prasinus 1.25 Nauplii 0.01 Results Zooplankton species distribution at the central surface does not differ statistically (p<0.05) from that near shores (Table 1) and mean densities in central columns and on shores follow similar patterns, both in time (Fig. 4) and in transect (Fig. 5); so, everything we say in relation to temporal and horizontal distributions will be applicable to the whole reservoir. Temporal distribution Figure 3. Space-time distribution of Chl-a concentration, in La Concepción reservoir, through time (A), transect (B) and depth (C). ential localization at the beginning of the cycle, at the intake or at the surface. The ANOVA application to density data gives significant F s for time in all and sundry species (Table 2), these results show a temporal variability bigger than horizontal and vertical variabilities. Polyarthra gr. vulgaris-dolichoptera, P. euryptera, Synchaeta gr. tremula-oblonga, Bosmina longirostris, Acanthocyclops robustus and Tropocyclops prasinus are present during mixing and stratification periods (Fig. 4). Except for B. longirostris, which has one density maximum during mixing and two during stratification, the species have one principal maximum during mixing or at the transition from stratification to mixing, and a secondary maximum during stratification.

5 161 Table 2. F s values for time, transect and depth resulting from ANOVA application to zooplankton densities of the central axis ( p<0.05, p<0.01) Time Transect Depth ROG: Polyarthra gr. vulgaris-dolichoptera P. euryptera Synchaeta gr. tremula-oblonga S. pectinata Anuraeopsis fissa Hexarthra mira Ascomorpha saltans Asplanchna gr. girodi-brightwelli Trichocerca similis Collotheca gr. pelagica CLA: Bosmina longirostris Diaphanosoma mongolianum Daphnia longispina Ceriodaphnia pulchella Ceriodaphnia reticulata COP: Acanthocyclops robustus Tropocyclops prasinus Nauplii Diaphanosoma mongolianum, Daphnia longispina, Anuraeopsis fissa and other less abundant species (Ceriodapnia pulchella, C. reticulata, S. pectinata, Hexarthra mira, Ascomorpha saltans, Asplanchna gr. girodi-brightwelli, Trichocerca similis and Collotheca gr. pelagica) are only or mainly present during stratification (Fig. 4), when they exhibit one maximum or, in the case of D. mongolianum, two maxima. The annual cycle follows this temporal succession pattern (Fig. 4). At the beginning of the mixing period, rotifers (Polyarthra gr. vulgaris-dolichoptera, P. euryptera and Synchaeta gr. tremula-oblonga) and copepods (A. robustus and T. prasinus) predominate; whereas cladocerans appear at very low density. In December, there is a general decrease in zooplankton populations. At the end of the mixing period, B. longirostris reaches high densities. Stratification begins with high densities of A. fissa and new peaks of copepods and S. gr. tremulaoblonga; afterward P. gr. vulgaris-dolichoptera, B. longirostris, D. mongolianum, D. longispina and a group of little abundant species (Ceriodaphnia reticulata, S. pectinata, H. mira, A. saltans and A. gr. girodibrightwelli) reach other peaks. About the middle of the stratification, densities of all populations decline. In the second part of this period, C. pulchella appears and other cladocerans, B. longirostris and especially the thermophyll species D. mongolianum, dominate. At the end of the period, copepod densities increase and other thermophyll species, T. similis and C. gr. pelagica, appear. Spatial distribution Horizontal distribution F s significance for transect (Table 2) have allowed us to differentiate: (A) A group of species (T. similis, C. gr. pelagica, C. pulchella and C. reticulata) occurring at very low density, with a non significant F s (p<0.05) and therefore homogeneous horizontal distribution. (B) Three groups of species with significant F s (p<0.05) and heterogeneous horizontal distribution which have been differentiated according the localization of the density maxima (Fig. 5): 1. Species (P. gr. vulgaris-dolichoptera, P. euryptera, B. longirostris, D, mongolianum, A. robustus, T. prasinus and the nauplii) with the horizontal maximum at the intake and a density gradient decreasing from intake to dam.

6 162 Figure 4. Temporal distribution of rotifers (ROT), cladocerans (CLA) and copepods (COP) species in the central axis and on the shores of the reservoir. M= Mixing period, S= Stratification period. Psp=Species of the genus Polyarthra (P. euryptera and P. gr. vulgaris-dolichoptera, the last one being the most); ST=Synchaeta gr. tremula-oblonga; AF=Anuraeopsis fissa; R1=Synchaeta pectinata+hexarthra mira+ascomorpha saltans+asplanchna gr. girodi-brightwelli; R2=Trichocerca similis+collotheca gr. pelagica. BL=Bosmina longirostris; DM=Diaphanosoma mongolianum, DL=Daphnia longispina, CP=Ceriodaphnia pulchella, CR=C. reticulata. AR=Acanthocyclops robustus, TP=Tropocyclops prasinus, NT=Nauplii.

7 Figure 5. Horizontal distribution of rotifers, cladocerans and copepods species in the central axis and on the shores of the reservoir. Abbreviation meanings appear below Figure

8 S. gr. tremula-oblonga, with the maximum in the centre of the transect. 3. Species (A. fissa, H. mira, S. pectinata, A. girodibrightwelli,a. saltans and D. longispina) with the maximum near the dam and a density gradient increasing from intake to dam. Vertical distribution Vertical distribution is different in each zone of the reservoir. At E1, in permanent mixing across the year, it is homogeneous for all species (Table 3). At E2 E6 (Fig. 6, Table 3), the distribution varies in each period of the annual cycle. During mixing, B. longirostris and A. robustus have significant F s (p<0.05) for depth and their vertical maxima are located in S. During stratification, most species (Table 4) present significant F s and their vertical maxima in 2S. During this last period, nauplii present their highest densities near the bottom and form an exception. Interspecific associations In the space defined by the first three components obtained from the principal component analysis (PCA) of zooplankton densities, we distinguish the following species groups (Fig. 7), in chronological order: 1. Species (P. gr. vulgaris-dolichoptera, P. euryptera, A. robustus, T. prasinus and nauplii) associated with component I, which present their highest densities at the beginning of mixing and at the intake, coinciding with time-space maxima of chlorophyll concentrations. 2. Species (A. fissa, S. gr. tremula-oblonga and C. reticulata) associated with component III, with density maxima at the beginning of stratification, coinciding with high oxygen and chlorophyll concentrations. 3. Species (D. longispina and rotifers S. pectinata, A. saltans, H. mira and A. gr. girodi-brightwelli) associated with component II, which appear in the first part of stratification, especially near the dam. 4. Species associated with component IV (D. mongolianum, C. pulchella, T. similis and C. gr. pelagica), all of them thermophyll species that appear during the second part of stratification. 5. B. longirostris, with high densities from the end of mixing period to the end of stratification, coinciding with the last three groups. ANOVA and PCA results have been used to construct the Figure 8 that show zooplankton species characterizing each period of the annual cycle and their spatial distributions. Discussion Similarities between central and littoral zooplankton distributions coincide with what we have found in chlorophyll distribution (Fernández-Rosado et al., 1994) and, like in this case, they are related to reservoir topography, water movements and lack of plants on the shores. These similarities allow us to consider the reservoir as a whole. In time, zooplankton community varies both in specific composition and in density distribution; however, in space, only population density changes. Spatial and temporal patterns of zooplankton distribution in this reservoir are closely related to chlorophyll and temperature. The relationship between zooplankton and chlorophyll distributions is shown by the correlation between chlorophyll concentration and population density (Table 4) and results from reciprocal interactions between zooplankton and phytoplankton (Kerfoot et al., 1988; Elser & Goldman, 1990; Brett et al., 1994; Vascancelos, 1994; Horn & Horn, 1995; Pejler, 1995; Vijverberg et al., 1997; Zingel & Ott, 2000). This reciprocity is particularly clear during stratification when, as is common in eutrophic and mesotrophic lakes (Bürgi et al., 1985, Luecke et al., 1990), the intense grazing of zooplankton causes a food decline, which is followed by a general decrease of zooplankton populations (Fig. 4). Temperature is another important factor affecting succession and distribution of zooplankton species. (Vasconcelos, 1994; Romo & Tongeren, 1995; Koksvik, 1995; Monchenko, 1996; Swadling et al., 2000) (Table 4). Species (P. gr. vulgaris-dolichoptera and S. gr. tremula-oblonga) negatively correlated with temperature, appear mainly at the beginning of the mixing (16 C). S. pectinata and H. mira, positively correlated with a coefficient significant at p<0.05, present their highest densities at the beginning of the stratification (19 22 C in epilimnion); whereas D. mongolianum, C. pulchella, T. similis and C. gr. pelagica, with a coefficient significant for p<0.01, are thermophyll species that especially appear in the second part of stratification (23 26 C in epilimnion). Relation between zooplankton and dissolved oxygen (Table 4) derives from the relation that the last one presents with chlorophyll (r=0.78) and temperature (r=0.89).

9 Figure 6. Vertical distribution of rotifers, cladocerans and copepods species in the mixing period and in the stratification period. 0m=surface, S=Secchi disk depth, 2S=twice this depth and B=bottom. Abbreviation meanings appear below Figure

10 166 Table 3. F s values for depth resulting from ANOVA application to zooplankton densities of E1 during the annual cycle and E2 E6, during mixing and stratification periods ( p<0.05, p<0.01) E1 E2 E6 Annual cycle Mixing Stratification ROT: Polyartrha gr. vulgaris-dolichoptera P. euryptera Synchaeta gr. tremula-oblonga S. pectinata Anuraeopsis fissa Hexartrha mira Ascomorpha saltans Asplanchna gr. girodi-brightwelli Trichocerca similis Colloteca gr. pelagica CLA: Bosmina longirostris Diaphanosoma mongolianum Daphnia longispina Ceriodaphnia pulchella C. reticulata COP: Acanthocyclops robustus Tropocyclops prasinus Nauplii Table 4. Spearman correlation coefficients of zooplankton species densities with environmental factors ( p<0.05, p<0.01) Chlorophyll-a Temperature Oxygen Polyarthra gr. vulgaris-dolichoptera P. euryptera Synchaeta gr. tremula-oblonga S. pectinata Anuraeopsis fissa Hexarthra mira Ascomorpha saltans Asplanchna gr. girodi-brightwelli Trichocerca similis Collotheca gr. pelagica Bosmina longirostris Diaphanosoma mongolianum Daphnia longispina Cerodaphnia pulchella C. reticulata Acanthocyclops robustus Tropocyclops prasinus Nauplios

11 167 Figure 7. Distribution of the zooplankton species in the space defined by the first three principal components; groups established among them are also represented. (% absobed variance). PV=Polyarthra gr. vulgaris-dolichoptera, PE=Polyarthra euryptera, ST=Synchaeta gr. tremula-oblonga, SP=Synchaeta pectinata, AF=Anuraeopsis fissa, HM=Hexarthra mira, AS=Ascomorpha saltans, AB=Asplanchna gr. girodi-brightwelli, TS=Trichocerca similis, CO=Collotheca gr. pelagica, BL=Bosmina longirostris, DM=Diaphanosoma mongolianum, DL=Daphnia longispina, CP=Ceriodaphnia pulchella, CR=C. reticulata, AR=Acanthocyclops robustus, TP=Tropocyclops prasinus, NT= nauplii. Competition is another factor that often structures zooplankton communities (Vasconcelos, 1994; Taleb et al., 1994; Pérez-Fuentetaja et al., 1996; Baca & Drenner, 1995; Nielsen et al., 2000). Important exploitative competition exists between cladocerans and rotifers, cladocerans being superior competitors because they have a bigger body size, are more efficient filter feeders and can also interfere mechanically with the rotifers that are swept into their branchial chamber (Brooks & Dodson, 1965; Gilbert, 1985, 1988; Hanazato & Yasuno, 1991). Nevertheless, in La Concepción reservoir, at the beginning of mixing (Fig. 4), rotifers are abundant and cladocerans are scarce. According to Kirk (1991), this scarcity may be a consequence of the decrease of phytoplankton ingestion rate that cladocerans suffer in presence of the high concentration of inorganic particles that are in suspension during mixing (García, 1995). During the second part of the stratification period, temperature increases (Fig. 2), chlorophyll concentration decreases (Fig. 3) and rotifers are dominated by cladocerans (Fig. 4), which intensifies their superiority when resources become limiting (Gilbert, 1988; Trubetskova & Lampert, 1995; Pejler, 1995). These cladocerans are B. longirostris and D. mongolianum, species of a very flexible feeding response that, besides algae, can ingest bacteria (Geller & Müller, Figure 8. Species characterizing zooplankton community during the mixing and stratification periods. Species with heterogeneous spatial distribution are included in the transect zone and in the column layer where they present their horizontal or vertical maximum; species with homogeneous spatial (horizontal or vertical) distribution appear below the corresponding model. Abbreviation meanings appear below Figure ; Bleiwas & Stokes, 1985; Kerfoot et al., 1988) and, in the case of D. mongolianum, with a great adaptation to high temperatures (Mourelatos & Lacroix, 1990). In zooplankton spatial distribution it is necessary to consider the influence of water stability and water movement. So, during stratification, the water stability determines the localization of the vertical maximum in 2S (Fig. 6), a depth that is included into thermocline during this period (Fig. 2). In horizontal distributions, the existence of gradients decreasing from intake to dam is commun in reservoirs and is related to transport and dilution associated to water renewal (Threlkeld,

12 ; Bailey, 1983; Armengol, 1984; Armengol et al., 1988; Urabe, 1990; Pinel-Alloul, 1995; Mayer et al., 1997). Species with the highest densities at the intake (Fig. 5) counteract this transport by means of a great mobility, as copepods, or by the combination of a high reproductive rate and certain mobility (Stross, 1996; Asaeda & Acharya, 2000; Razlutskij, 2000), like cladocerans B. longirostris and D. mongolianum or rotifers P. gr. vulgaris-dolichoptera and P. euryptera, the most mobile rotifers in this reservoir because of their fins. The rest of the rotifers are mainly located in the centre of the transect or near the dam (Fig. 5). D. longispina also reaches maximum densities near the dam, even though this large cladoceran could counteract water movement and dominate species concentrated at the intake (Demott & Kerfoot, 1982; Gilbert, 1988; Santer, 1993). The distribution of this last species is similar to that described by Urabe (1990) for D. galeata in a Japanese reservoir and, like in this case, may result from predation by fishes (Taleeb et al., 1994; Baca et al., 1995; Vijverberg & Boersma, 1997; Horppila, 1999; Nielsen et al., 2000), which concentrate at the intake of the reservoir (Lucena et al. 1983). Acknowledgements We are grateful to Ramón Hidalgo from the Computer Centre at Málaga University and José Ángel Gálvez, Fidel Echevarría and all the members of the Ecology Department from the same University, for their information and help in the sampling. This work was supported by the Junta de Andalucía and the C.I.C. y T. of the Spanish Ministry of Education and Science. References Algarra, P., Gradientes espaciales de pigmentos clorofilicos como indicadores de estrés en sistemas acuáticos y sedimentarios. Tesis de Licenciatura. Universidad de Málaga: 97 pp. Arcifa, M. S., E. A. T. Gomes & A. J. 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